The present invention relates to a multilayer printed wiring board for various kinds of electronic equipment and its manufacturing method.
In recent years, as a higher degree of functionality and components density is required of electronic equipment such as personal computers, mobile telephones, video cameras and the like, it has become necessary for the electronic components used in the electronic equipment, among which semi-conductor devices occupy a central position, to be smaller in size, larger in packing density, higher in speed and/or higher in I/O pin count.
As a result, it has become also necessary for multilayer printed wiring boards to be enhanced in the ability to accommodate wiring and in the surface mounting density of components. In addition, as the diameter for each respective land for soldering becomes smaller, it has become necessary for the reliability in adhesion between the board and the components mounted thereon to be enhanced. More specifically, it has been required of a printed wiring board to have the capability of meeting both requirements of a high density as exemplified by a ball grid array (referred to as BGA hereafter) of a 0.5 mm pitch and a small diameter land of 0.3 mm or less. For example, such a printed wiring board that shows excellent resistance to mechanical stresses such as an impact caused by a drop test and the like has been increasingly demanded.
In order to meet the foregoing requirements, such a prior art multilayer printed wiring board as described below is proposed. The prior art multilayer wiring board comprises an inner layer material and a photosensitive resin or a film-like insulating layer, formed on each respective surface of both sides of the inner layer material. The inner layer material includes a resin multilayer printed wiring board and the respective layers in the resin multilayer printed wiring board are connected to one another electrically by means of interstitial via holes (IVH). The photosensitive resin or insulating layer is formed on each respective surface of both sides of the inner layer material by coating or lamination. Non-through holes are formed in the inner layer material and the layers inside thereof are electrically connected with one another by means of a metal plating method.
Next, a description is given to a manufacturing method of the foregoing prior art multilayer printed wiring board.
FIG. 3 shows how the prior art multilayer printed wiring board is prepared. In FIG. 3, an insulating layer 12 formed of a photo-sensitive resin and the like is disposed on the outer most layer of the prior art multilayer printed wiring board 15 by means of coating or lamination. The prior art multilayer printed wiring board 15 comprises a conductive pattern 11 for outer layer, a resin insulating layer 12, an inner layer material 13, a non-through hole 12a and a surface via hole (SVH) 11a. The inner layer material 13 includes an insulating substrate 14, a conductive pattern 14a for inner layer material, a copper foil 14d and a conductive paste 14b for inner layer material. The insulating substrate 14 is prepared from a prepreg 14c. The surface via hole 11a is formed by applying a metal plating to the non-through hole 12a formed in the resin insulating layer 12. In preparing the surface via hole 11a, the non-through hole 12a is formed by the use of such a method as a light exposure-development method, a laser irradiation method or the like applied to the resin insulating layer 12. The multilayer printed wiring board 15 has a conductive pattern inside and outside thereof, respectively. Now, a description is given to a manufacturing method of the multilayer printed wiring board structured as in above.
First, a hole making process is applied to a prepreg 14c in the step of FIG. 3(a). A conductive paste 14b is filled in the hole formed as in above. Then, a copper foil is superimposed on the prepreg 14c and heat pressed, thereby having the copper foil attached by adhesion to the prepreg 14c, in which a conductive paste 14b is filled. Accordingly, a copper laminated board with a copper foil disposed by lamination on both sides of an insulating substrate 14, respectively, is obtained. Thereafter, a conductive pattern 14a for inner layer material is formed by the use of such publicly known methods as a screen printing method, a photograph method and the like. Thus, the insulating substrate 14 with the conductive pattern 14a put in place on both sides thereof, respectively, is obtained.
In the step of FIG. 3(b), a prepreg 14c is prepared by filling a conductive paste 14b in a hole. The prepreg 14c filled with the conductive paste 14b is superimposed, respectively, on both sides of an insulating substrate 14 with a conductive pattern 14a formed on both surfaces thereof, respectively. Further, a copper foil 14d is superimposed on the surface of the prepreg 14c filled with the conductive paste 14b. Then, those laminates are heated by a heat press and a pressing force is applied thereto.
In the step of FIG. 3(c), the publicly known screen printing method, photographic method or the like is applied to the copper foil 14d put in place in the foregoing step of FIG. 3(b), thereby allowing a conductive pattern 14a to be further formed, respectively, on both surfaces of an inner layer material 13, which is consequently obtained as FIG. 3(c) shows.
Next, in the step of FIG. 3(d), a resin insulating layer 12 formed of a photosensitive type resin and the like is applied onto the inner layer material 13 so as to remain in a semi-hard state or the resin insulating layer 12 is laminated on the inner layer material 13.
Then, in the step of FIG. 3(e), a non-through hole 12a is formed at a predetermined position by the use of an exposure-development method, a laser irradiation method or the like.
In the step of FIG. 3(f), a conductive pattern 11 is formed on the resin insulating layer 12 and a surface via hole 11a is formed in the non-through hole 12a by metal plating. The surface via hole 11a has the function of connecting electrically between an inner layer conductive pattern and an outer layer conductive pattern. As a result, a multilayer printed wiring board 15 is obtained.
Afterwards, an application of a solder resist is performed by the use of such publicly known methods as a photographic method and the like and then working on the outside shape and the like follow.
With the foregoing prior art multilayer printed wiring board, the inner layer material has an interstitial via hole (IVH) formed on each respective layer at an arbitrary position and further has a small non-through hole ranging from about 50 xcexcm to 100 xcexcm in diameter formed on the outer layer thereof. Accordingly, the prior art multilayer printed wiring board has shown the excellent ability to accommodate wiring and to perform high density surface mounting. However, on the other hand, the prior art multilayer printed wiring board has revealed a flaw of weak adhesion between the conductive pattern 14a for outer layer and the resin insulating layer 12. In recent years, as a higher degree of ball grid array integration and a higher degree of components density are required, a land for soldering has become smaller and smaller in diameter and a requirement for enhanced adhesion between the conductive pattern 14a for outer layer and the insulating substrate 14 has been made.
More specifically, since a plated layer formed on a resin by metal plating shows weak adhesion therebetween, with the prior art multilayer printed wiring board 15, in which a conductive pattern 11 is formed on a resin insulating layer 12 by metal plating, the adhesion between the conductive pattern 11 formed on the resin insulating layer 12 and the resin insulating layer 12 is weak. As a result, when components are mounted densely on the multilayer printed wiring board 15 with a consequent use of small diameter lands for soldering, the conductive pattern 11 is liable to be peeled off the resin insulating layer 12 due to a mechanical stress, in particular.
In addition, the insulating substrate 14 constituting the inner layer material 13 and the insulating layer 12 constituting the outer most layer are different from each other in the hardening process, resulting in a big difference in physical properties existing therebetween. Therefore, the adhesion between the inner layer material and the outer most layer becomes weak. Also, there is the danger of causing cracks and delamination to be created between the inner layer material and the outer most layer due to the heat produced at the time of soldering in the step of mounting components since there exists a difference therebetween in thermal expansion coefficient.
The present invention provides a multilayer printed wiring board with such features as enhanced adhesion between the outer layer conductive pattern and the insulating layer and excellent mounting reliability against a mechanical stress imposed on highly integrated/densely populated components such as a ball grid array (BGA) of a 0.5 mm pitch and the like while maintaining the excellent features associated with the prior art multilayer printed wiring board in terms of the ability to accommodate wiring and the high density surface mounting.
A multilayer printed wiring board of the present invention comprises:
(a) an inner layer material comprising an insulating substrate, a inner conductive pattern which is formed of a metal foil disposed on each respective surface of both sides of the insulating substrate, and an interstitial via hole disposed on the insulating substrate;
(b) an insulating resin disposed on each respective surface of both sides of of the inner layer material;
(c) an outer conductive pattern disposed on the surface of the insulating resin; and
(d) a surface via hole electrically connecting between the inner conductive pattern and the outer conductive pattern,
in which the interstitial via hole connects electrically between respective inner conductive patterns among the plurality of inner conductive pattern-saterial, and
the outer conductive pattern is formed of the metal foil of a metal foil with insulating resin, in which metal foil with insulating resin comprises the insulating resin and the metal foil adhered to the insulating resin.
A manufacturing method of a multilayer printed wiring board of the present invention comprises the steps of:
(a) producing an inner layer material,
the inner layer material comprising
an insulating substrate,
an inner conductive pattern formed of a metal foil disposed on each respective surface of both sides of the insulating substrate and
an interstitial via hole disposed on the insulating substrate;
(b) superimposing a metal foil with insulating resin on each respective surface of both sides of the inner layer material, in which the metal foil with insulating resin has an insulating resin and a metal foil adhered to the insulating resin,;
(c) applying a pressing force to the inner layer material and the metal foil with insulating resin superimposed on each respective surface of both sides of the inner layer material while heat being applied thereto, thereby allowing the insulating resin to adhere to the inner layer material;
(d) forming a non-through hole in the metal foil with insulating resin by working on the metal foil with insulating resin;
(e) forming a conductive pattern for outer layer by working on the metal foil exposed on the surface; and
(f) connecting electrically between the outer conductive pattern and the inner conductive pattern.
As a result, a multilayer printed wiring board with the exceptionally excellent ability to accommodate wiring is realized. In addition, the adhesion between the outer conductive pattern and the substrate material is greatly enhanced. Therefore, even with a multilayer printed wiring board having respective lands provided with a small diameter is allowed to realize a high degree of components mounting reliability.